1. Field of Use
This invention relates generally to apparatus for locking and unlocking the freely oscillatable rear axles of rough terrain mobile cranes and, in particular, to selectively operable override control means in such apparatus to effect axle unlocking.
2. Description of the Prior Art
Some rough terrain mobile hydraulic cranes comprise a mobile lower section, including a chassis or carrier frame having rubber-tired ground-engaging wheels mounted on vertically oscillatable axles. A rotatable upper section, including a boom, is mounted on the carrier. The oscillatable axles, usually the rear axles, are free to move up and down with the contour of uneven ground as the crane is driven thereacross and such oscillation keeps the wheels in contact with the ground on uneven terrain to better support the crane when it is to be operated. Each such axle pivots about a central point and, for example, the right rear wheel may be high while passing over a bump, in which event the left rear wheel will be low relative to the carrier frame. When the crane is "working on rubber", i.e., without its outriggers extended, it is possible for tipping of the crane to occur as a load is swung or lifted over the side of the crane by the boom on the rotated upper section.
Two hydraulic cylinders mounted between the ends of the axle and each side of the frame to permit axle oscillation must be locked while lifting loads in other than the front area of the machine in order to prevent tilting of the carrier frame. Accordingly, locking means are provided for locking the cylinders for the oscillatable axles in down position when lifting over the side of the crane is to be carried out. One type of prior art locking means comprises a manually movable member which is placed in an interfering relationship with relatively movable components of an oscillating axle when locking is required. Another type of prior art locking means comprises hydraulically powered and controlled locking cylinders or rams which are actuatable to effect axle locking and unlocking automatically in response to the rotary position of the crane upper section relative to the carrier frame. In one embodiment of the latter prior art means a locking cylinder having its housing connected to the frame has a piston rod which is connected to and oscillates with the oscillatable axle as the wheel moves over uneven ground. Two such locking cylinders on opposite sides of the crane are interconnected through and controlled by a hydraulic locking valve.
Generally speaking, locking is accomplished automatically by actuating an electric cam controlled switch while the crane is in the travelling mode (boom over front), which switch in turn may activate a hydraulic solenoid valve. The solenoid valve directs the pilot pressure to open the hydraulic locking valve in the cylinder circuit to let hydraulic oil move freely between the two cylinders and sump.
The above function can also be accomplished more simply by mechanically actuating a spool operated lock valve by the rotating motion of the crane upper structure. The lock valve is kept in release position while the machine is in travelling position and automatically locked when the upper is rotated. Such a lock valve has a movable spool which is actuated by a cam on the crane upper to either unlocked or locked position. When the crane upper section is centered with the boom over front on the carrier frame, the valve is unlocked by the cam position and oil is able to flow freely between the two lockout cylinders which are then able to ocillate with the axles in response to ground contours. When the crane upper section is off-center, as when handling a side load, the valve is locked as a result of cam position and the locking cylinders then lock in the last position they were in so as to hold the rear end of the carrier frame in horizontal position. In one such prior art arrangement, the locking valve includes a pair of ball-type check valves operated by a vertically shiftable two-position valve stem which, when depressed by the cam, allows the check valves to open and unlock the cylinder, and when released by the cam, causes the check valves to close to prevent free oil flow and lock the cylinders. However, sometimes it is desirable to be able to unlock the oscillatable axles, even while the upper section is off-center, so that the carrier can be repositioned and override means are provided in some prior art axle locking systems. For example, for pick and carry operation or moving the machine around, often the boom is not in the front position. Therefore, in automatically actuated systems, the cylinders stay in locked position preventing free oscillation or repositioning of the axle. Under these conditions an override control is necessary so that the operator can make on the spot adjustment to position the axle correctly.
Override control can be accomplished in the aforementioned system having an electric cam controlled switch wherein when electric signal from the cam switch is available. However, the system becomes complicated since it must use some form of hydraulic pilot interface to actuate the cylinder locking valves. The mechanical system described above is simple, but is not well suited for inclusion of override control means.